I've seen the PM but, unfortunately, I don't seem able to attach a pdf document in the reply. I think, as ever, there may be issues associated with terminology - gagging order, confidentiality clause, non disclosure condition, settlement agreement etc and the (sometimes) subtle (sometimes not so subtle) legal differences between them. This is then further compounded by Cllrs perhaps not fully understanding the differences (or even that differences may exist between them) and also using the wrong phrase to describe the situation they actually have.
I think you'd need to try and determine EXACTLY what it is you have discovered, on what basis it was established, who provided the legal advice (and what was it) and what does the "agreement" seek to actually DO.
2 things spring immediately to mind - As Dave says below, sounds like a load of old twaddle since almost all council business is open source and since you were not on the council at the time, you cannot be bound by any agreement that was entered into prior to your appointment. Some may argue that it was a corporate decision of the PC (if that was the case) and so you, as a Cllr, are bound by it, but that argument is without foundation if it exists prior to your appointment.
The absolute lunacy of the situation I found myself in was that the PC, before I was elected, had entered into a settlement agreement with the former clerk and refused to even acknowledge that a settlement agreement had been entered into.
So any new Cllr would have no knowledge of its existence, or the content / subject, or the supposed limitations! It was pure comedy - Don't tell them your name Pike!
In that case. it was a "Settlement Agreement" which included elements which precluded either party discussing the content... Of course that entirely suited the PC because it gave them top-cover to attempt to conceal their appalling HR procedures and the fact that the employee had been pulling their pants down for years.
No wonder the PC wanted to keep it quiet - oh well, what a shame, that didn't happen...